FIG. 11a is a perspective view schematically showing an example of an antenna structure. FIG. 11b is an exploded view schematically showing the antenna structure. FIG. 11c shows the antenna structure shown in FIG. 11a when viewed from the bottom side. The antenna structure 1 includes an antenna 2. The antenna 2 is mounted in a non-ground region Zp of a circuit board 3. That is, a ground region Zg in which a ground 4 is formed and the non-ground region Zp in which the ground 4 is not formed are arranged next to each other on the circuit board 3 such that the non-ground region Zp is disposed on one end of the circuit board 3. The antenna 2 is mounted in the non-ground region Zp of the circuit board 3. As a board of a non-ground region, for example, a glass-epoxy board whose both surfaces are not coppered can be used.
The antenna 2 includes a dielectric base member 6, a feed radiation electrode 7, and a non-feed radiation electrode 8. The dielectric base member 6 is a rectangular parallelepiped (a rectangular column). On the upper surface of the dielectric base member 6, the feed radiation electrode 7 and the non-feed radiation electrode 8 are arranged with a space therebetween. The feed radiation electrode 7 and the non-feed radiation electrode 8 are electromagnetically coupled to each other to produce a multiple-resonance state. In addition, on a side surface 6a, which is an outer side surface of the dielectric base member 6 along an edge of the one end of the circuit board 3 near a top side remote from the ground 4, a feed end Q of the feed radiation electrode 7 and a short end S of the non-feed radiation electrode 8 are formed.
In addition, in the non-ground region Zp of the circuit board 3, a feed electrode 10 (10B) connected to the feed end Q of the feed radiation electrode 7 is provided. The feed electrode 10 (10B) is an electrode pattern that extends along side surfaces of the dielectric base member 6 from a portion connected to the feed end Q of the feed radiation electrode 7 toward the ground region Zg. An end of the feed electrode 10 (10B) near the ground region Zg is connected to a high-frequency circuit 12 for radio communication of a radio communication apparatus. In addition, in the non-ground region Zp of the circuit board 3, a ground connection electrode 11 (11B) connected to the short end S of the non-feed radiation electrode 8 is provided. The ground connection electrode 11 (11B) is an electrode pattern that extends along side surfaces of the dielectric base member 6 from a portion connected to the short end S of the non-feed radiation electrode 8 toward the ground region Zg. An end of the ground connection electrode 11 (11B) near the ground region Zg is grounded to the ground 4.
In the antenna structure 1, for example, when a signal for radio communication is supplied from the high-frequency circuit 12 for radio communication to the feed radiation electrode 7 via the feed electrode 10 (10B), the feed radiation electrode 7 resonates. The non-feed radiation electrode 8, which is electromagnetically coupled to the feed radiation electrode 7, also resonates. Thus, the feed radiation electrode 7 and the non-feed radiation electrode 8 produce a multiple-resonance state, and a signal is transmitted wirelessly.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-217631
For example, in the antenna structure 1 shown in FIG. 11a, the feed radiation electrode 7 and the non-feed radiation electrode 8 are mainly provided on the upper surface of the dielectric base member 6. Thus, electromagnetic fields radiated from the feed radiation electrode 7 and the non-feed radiation electrode 8 are concentrated on the upper surface of the dielectric base member 6. Thus, a problem occurs in which a Q-value, which is an antenna characteristic, is likely to increase and in which a frequency bandwidth for radio communication is likely to decrease. In addition, there is a problem in which antenna characteristics deteriorate due to increases in conductive loss and dielectric loss.
In addition, in order to realize an electrical length to achieve a required resonant frequency, slits may be formed in the feed radiation electrode 7 and the non-feed radiation electrode 8. However, since the feed radiation electrode 7 and the non-feed radiation electrode 8 are provided on the upper surface of the dielectric base member 6, that is, provided on a single surface of the dielectric base member 6, the feed radiation electrode 7 and the non-feed radiation electrode 8 have limited electrode areas. Thus, when a slit-formed area within an electrode unit area of each of the feed radiation electrode 7 and the non-feed radiation electrode 8 increases, the electrode width of a current path of each of the feed radiation electrode 7 and the non-feed radiation electrode 8 decreases. This causes a problem in which conductive loss increases in the feed radiation electrode 7 and the non-feed radiation electrode 8. In addition, as the slit-formed area increases, a configuration of each of the feed radiation electrode 7 and the non-feed radiation electrode 8 becomes more complicated.
In addition, metal or high-dielectric materials (for example, human fingers or the like) are often above the antenna 2. In this case, radio waves radiated from the feed radiation electrode 7 and the non-feed radiation electrode 8 are blocked by the metal or high-dielectric materials. This causes a problem in which antenna gain decreases. In addition, a problem occurs in which changes in impedances of the feed radiation electrode 7 and the non-feed radiation electrode 8 caused by a distance change of an object regarded as a ground deteriorate antenna characteristics.